In the search for high performance materials, considerable interest has been focused upon carbon fibers. Industrial high performance materials of the future are projected to make substantial utilization of fiber-reinforced composites, and carbon fibers theoretically have among the best properties of any fiber for use as high strength reinforcement. Among these desirable properties are corrosion and high temperature resistance, low density, high tensile strength, and high modulus.
Polymeric materials heretofore commonly have been selected as the matrix material in which the relatively delicate carbon fibers are incorporated to form high performance materials with the carbon fibers serving as a light weight fibrous reinforcement. Epoxy resins most frequently have been selected as the matrix material. Additionally, the use of polyimides as the matrix material has been proposed primarily because of the ability of the polyimides to withstand even higher temperatures during use.
In order to improve the handleability of the carbon fibers without undue fiber damage during the formation of composite articles, it has been the common practice to apply a flexible size or finish to the surface of the same. See, for instance, the epoxy size disclosed in U.S. Pat. Nos. 3,837,904; 3,914,504; and 3,957,716. The sized carbon fibers bearing the epoxy coating are commonly incorporated in an epoxy matrix resin and a rigid composite article is formed upon curing.
There has remained a need, however, for a size or finish for use with carbon fibers which is capable of enhancing the handleability of the fiber which at the same time improve the impact resistance of the composites employing the fiber as a reinforcing agent.